The use of siloxane resins as a desired resin additive in forming protective coatings is well known in the art. In certain applications, the addition of a siloxane ingredient, e.g., a siloxane resin, is known to contribute properties of flexibility, impact and weather resistance to the resulting cured film coating. In one such example, a polysiloxane resin is combined with an epoxy resin to provide improved properties of impact resistance, flexibility, corrosion resistance, and weatherability to the resulting epoxy resin-based film coating.
Typically, siloxane resins used in this capacity react with a base resin material, e.g., an epoxy resin, by an acid or base catalyzed hydrolysis of the siloxane resin and an aminosilane, followed by condensation of the resulting silanol groups formed during hydrolysis and reaction of amine with epoxy. This reaction mechanism is one that is initiated by the presence of moisture conducted in the presence of an amine, and driven to completion by evaporation of alcohol formed during the hydrolysis reaction. While such known epoxy-polysiloxane coating compositions are useful in forming protective coatings providing a degree of coating hardness, flexibility, impact resistance, weatherablity, and corrosion and chemical resistance to an underlying substrate, such coating properties occur or develop only after the passage of a particular drying or curing time. The need to provide coatings having reduced volatile organic content (VOC), to meet certain regulatory requirements, has necessitated the use of lower molecular weight resins in the formulation of such coatings.
A disadvantage, however, with using such lower molecular weight resins is that the desired above-noted coating properties can only be acquired by increasing the cross-link density of these resins, which takes longer and requires a corresponding longer drying or curing time and/or increased energy input (for example relating to external heating equipment that may be use to improve the cure time) when compared to coating compositions prepared by using higher molecular weight resins. Further, the increased cross-link density can result in the coating having a reduced degree of flexibility.
In an example known epoxy-polysiloxane coating composition, the cross-link density of the combined ingredients is attained by the reaction methodology noted above; namely, by hydrolytic condensation of the siloxane ingredient and reaction of amine with epoxy resin. While these ingredients are known to provide a protective coating having desired chemical and mechanical properties, while also meeting reduced VOC requirements, the drying time for such coating may be unsuited for certain applications calling for quickened or reduced drying times.
It is, therefore, desired that modified siloxane compositions be formulated that are capable of providing a degree of coating flexibility, hardness, impact resistance, weatherablity, and corrosion and chemical resistance that is the same as or better than that of known epoxy-polysiloxane coating compositions, while at the same time providing such properties within a reduced drying or cure time and over a broad temperature range that can include ambient temperature if so desired. If formulated to provide reduced curing time and curing at ambient temperature, modified siloxane compositions of this invention provide a further advantage of avoiding the need to use external heating devices for curing. It is also desired that modified siloxane compositions of this invention be formulated to provide such desired chemical and mechanical properties within reduced drying and cure times using readily available material and without the need for special processing using exotic techniques or equipment.